An opto-electronic package is a housing that provides protection and support for both active and passive opto-electronic optical devices contained within it. The devices and their ability to interconnect with other components define an optical-electronic circuit and determine the functionality of the package. The packaging may also include a means of connecting the internal optical circuitry with the external world. Typically, this is accomplished by conventional electrical means or optical fiber.
Circuits that permit optical signaling offer substantial advantages over circuits that are limited to only electronic signaling. Optically modulated signals can carry more data, and at higher speeds than electrical signals. Further, optical signals are immune to electromagnetic interference and can be employed over longer distances with minimal attenuation. Optical signals, unlike electronic signals, can interpenetrate each other spatially without interfering with each other.
It is expected that optical signaling will become an increasingly important technology in interconnecting internal computer components. Computers are expected to emerge based on neural network architectures, where computer power is based on quantity of interconnect rather than quantity of memory. Optical signaling is expected to play a key part in these neural designs. There is a growing list of optical devices that need to be interconnected. The list of devices includes active discrete devices, passive discrete devices, and photonic integrated circuits. Examples of active discrete devices are photodetectors, modulators, and active waveguides that switch, control, amplify, emit light or detect light. Examples of discrete passive devices include straight and bent waveguides, splitters that branch the flow of light, and couplers that channel specific wavelengths. The photonic integrated circuits combine functions and are usually fabricated on a specific substrate such as InP or GaAs. Examples might include laser or detector arrays, integrated optical preamplifiers, lasers with integrated beam expanders or tunable DFB's.
Electronic integrated circuits have been electrically interconnected into large high-density circuits using a multi-chip module packaging approach as described by the International Electronics Packaging Society in "Multi-chip Modules", Proceedings of the IEPS Technical Conference, The IEPS Society, Wheaton, Illinois, pps. 3-55 and 401-470, (1990). These modules consist of mechanically precise substrates that support tape automated bonded (TAB) or another type of unencapsulated electronic chips. Precision deposited high frequency electronic signal lines are buried in the substrate, providing the means of electrically interconnecting the chips. The modules offer dense, high chip count packaging for high frequency electronic circuit applications. The modules, however, only provide electrical interconnect. There is no optical interconnect capability. The current invention discloses a means for optically interconnecting optical devices and a means for implementing the interconnect in a multi-chip modular form.
Present optical interconnect technology is either fiber based, planar waveguide based, or free-space transmission based. The current approaches to interconnecting chips within a package have specific disadvantages making them costly, inefficient or difficult to implement. A fiber based interconnect is most common, providing the basis for fiber optic links between separate packaged modules. These links are usually separated by large distances, since fiber offers the best coupling efficiency for distant optical interconnect. But, while fiber is suited for coupling at a distance, it is rarely if ever used for connecting optical devices within the same package. A lens is required to magnify the elements of the laser array to mode match with the larger core size of optical fiber. Dimensional restrictions prohibit the placement of optical fiber closer together than the distance of its own diameter. The fiber tips need special shaping into lenses or optical flat cleaves that can not be easily formed for extremely short fiber lengths. Installation of short fibers is very difficult, requiring a separate +/-0.5 .mu.m tolerance and stable alignment at each end of the fiber.
A planar waveguide based interconnect, as described in C. H. Henry, G. E. Blonder, and R. F. Kazarinov, "Glass Waveguides on Silicon For Hybrid Optical Packaging", J. Lightwave Technology, vol. 7 page 1530 (1989) has recently been described, but development of the technology is slow due to the difficulty of connecting the planar waveguides to the device chips. Further, there is a considerable amount of optical loss within a planar waveguide because optical quality is not as good as fiber.
Planar waveguides employ a substrate, such as silicon or glass, that has patterned light pathways on or in its surface. With glass substrates, planar waveguides channel and confine light with patterns of dopants that locally raise the index of refraction within the pattern of guides. With silicon substrates, light is usually guided through silica glass ridge planar waveguides that are formed on the silicon surface.
Unlike fiber, a planar waveguide based interconnect can be used to connect separate devices within the same package. The optical devices are either grown into wells on the substrate or mounted that way, such that the active elements of the devices will butt-couple their light into or out of the ends of the planar waveguides.
One major disadvantage of planar waveguide based interconnect is that the optical coupling efficiency through the butt coupled edges is worse than that for fiber based interconnect. The mechanical tolerances are tight in an attempt to reduce the gap between the planar waveguide and the device. The larger the gap, the worse the coupling. Regrowth to "fill" the gaps has been attempted, but processing is difficult since the dissimilar silicon based planar waveguides and III-V optical device materials are present simultaneously during processing. Removal of optical devices for repair or replacement is virtually impossible.
Like planar waveguides, free space transmission can provide signal transmission between optical devices within the same package. This technology does not utilize a planar waveguide but rather projects the light through space to its destination. One version of this interconnect, as described by S. Esener and S. H. Lee in "Free Space Optical Interconnects for Microelectronics and Parallel Computing", SPIE Proceedings of the Technical Conference, vol. 1178 page 84 (1989), projects light onto a hologram which then reflects it back to specific receivers. The hologram is located in a separate plane above the optical devices. Another form of this interconnect, as described by D. Z. Tsang, "Free-space Optical Interconnects", SPIE Proceedings of the Technical Conference, vol. 994 page 73 (1988), permits transmission between surface emitters and detectors located on separate planes or boards held parallel to each other. Tiny lenses at each element beam the light across to the neighboring board.
There are several drawbacks to the holographic, reflecting, and parallel board type free space interconnect. All require either surface emitting lasers, detectors or other devices, but at present most optical devices interconnect through their edges. Coupling efficiency is poor owing to the difficulty in maintaining parallelism between the reflector plane and the circuit board plane. The placement of tiny microscopic projection lenses at each optical element is costly.